The subject of the present invention is a process for the regeneration of a hydrocarbon reforming catalyst which has been deactivated due to buildup of carbonaceous material on the catalyst. More specifically, the present invention provides a method for regenerating a coke deactivated reforming catalyst by a controlled low temperature carbon-burn procedure which results in the catalyst having activity, selectivity and stability characteristics approximating, or which are even better than, the catalyst prior to the buildup of carbon. The reforming catalysts intended to be regenerated by the process of the present invention comprise a molecular sieve material and platinum.
Platinum containing catalysts are widely used in the oil refining and petrochemical industries, and are particularly important in a reforming process where paraffins, olefins and naphthenes are converted to aromatic compounds. Conventional reforming catalysts typically include one or more metals, most typically platinum, dispersed on a base, and may also include a binding agent for adding physical support to the base, and chloride to provide an acidic function. Typically, the catalyst base is alumina, but recently molecular sieve based catalysts have been found to be effective for reforming reactions.
Catalytic compositions containing zeolites are well known in the industry and recently the use of L-zeolites in combination with other specified catalytic components have been found to be particularly preferred for reforming. The aromatic compounds produced by such a catalytic conversion are valuable to a refiner due to their higher octane rating, and may be recovered from the reforming product for further processing and reaction in the petrochemical industries. The L-zeolite catalysts are particularly effective for converting C.sub.6 and C.sub.7 non-aromatic hydrocarbons which normally boil between 125.degree. F. and 225.degree. F., to benzene and toluene.
In the reforming process, a hydrocarbon feedstock is passed through a catalyst-containing reactor in the presence of hydrogen at an elevated temperature. In the reactor and upon contact with the reduced or activated catalyst, some of the paraffins, olefins and naphthenes in the feedstock react to form a more desired, higher octane aromatic product. In the course of typical reforming operations, the catalysts will typically become deactivated due to the deposition of carbonaceous material or "coke" upon the catalyst, and/or sintering or poisoning of the catalytic metal particles. In commercial practice, the deactivated catalyst will then be regenerated by a process which may include as a step the exposing of the catalyst to an oxygen containing gas at elevated temperature (typically 950.degree. F. final burn temperature) to burn the carbonaceous deposits accumulated on the catalyst. However, prior to our present invention, burning the coke off the catalyst at elevated temperatures caused the catalytic metal to agglomerate, which significantly lowered the activity and thus worsened the performance of the catalyst. Consequently, specific steps directed to the redispersion of the catalytic metal on the base were necessary. The redispersion steps typically comprise contacting the catalyst with a chloride containing gas in the presence of oxygen and water vapor, and are well known in the art.
In U.S. Pat. No. 4,493,901 issued Jan. 15, 1985 to Bernard et al., a method of regenerating a catalyst comprising platinum and a non-acidic L-zeolite is described, whereby the catalyst is subjected to a multi-step process, the first step being exposure to a hydrogen environment, followed by a carbon burn step, and then a calcination step followed specifically by an oxychlorination step. The regeneration method of Bernard et al. utilizes a hydration step following the oxychlorination step. In the hydration step, a quantity of water is added to the cooling air.
In U.S. Pat. No. 4,810,683, issued Mar. 7, 1989, to Cohn, a method is described which utilizes a halogen or halogen containing gas during the carbon burn step. The burning step of the process of U.S. Pat. No. 4, 810,683 occurred necessarily in the presence of a halogen or halogen containing gas and at a temperature of from 300.degree. C. to 600.degree. C.
U.S. Pat. No. 3,986,982, issued Oct. 19, 1976, to Crowson et al., describes a method for regenerating a deactivated platinum group metal-loaded zeolites by burning off deposits on the catalyst with a stream of inert gas and oxygen at a temperature controlled to a maximum of 550.degree. C. Example 1 shows a stepwise burn procedure to 530.degree. C. The water content was held at 100-130 ppm. This was followed by treating with a stream of an inert gas containing from 0.5 to 20.0 vol % oxygen and from 5 to 500 ppmv chlorine as chlorine gas. The resultant catalyst is purged to remove residual oxygen and chlorine and then reduced with hydrogen.
U.S. Pat. No. 4,359,400, issued Nov. 16, 1982, to G. R. Landolt et al., describes a process whereby multimetallic platinum containing catalysts are contacted with oxygen containing gas at elevated temperatures to burn off coke and then treated with dry, oxygen-free hydrogen halide. The catalyst is then treated with Cl.sub.2 containing gas which is oxygen and water-free.
U.S. Pat. No. 4,719,189, issued Jan. 12, 1988 to A. S. Krishnamurthy describes a process for rejuvenating a coke-deactivated nobel metal containing zeolite catalyst. The process comprises contacting the catalyst with a mixture of oxygen and sulfur dioxide, followed by catalyst reduction. Temperatures from 752.degree.-840.degree. F. are preferred.
U.S. Pat. No. 4,645,751, issued Feb. 24, 1987, to S. B. McCullen et al., describes a process for rejuvenating a highly silicious (silica-alumina ratio of at least 20) Pt zeolite catalyst wherein the Pt has become agglomerated. The agglomerated catalyst is first reduced with hydrogen. Following reduction the catalyst is treated with hydrogen chloride and oxygen in an inert gas carrier at 400.degree.-450.degree. C.
U.S. Pat. No. 4,752,595, issued Jun. 21, 1988, to S. B. McCullen et al., describes a process for regenerating a nobel-metal containing zeolite catalyst. The process consists of a coke burn step followed by a low temperature reduction step (212.degree.-482.degree. F.) Oxidizing conditions for the coke burn include temperatures ranging from 392.degree.-1004.degree. F., preferably 752.degree.-950.degree. F.
U.S. Pat. No. 4,657,874, issued Apr. 14, 1987, to W. S. Borghard et al., describes a method where Pt on a highly silicious zeolite is redispersed. The process comprises a coke burn followed by oxychlorination. The oxygen burn step is done under conditions that are sufficiently mild to prevent any alteration in the crystal structure of the zeolite. Example 6 uses a temperature of 850.degree. F. This is then followed by treating with a stream of inert gas containing 6 to 15 Torr of chlorine gas and water vapor such the molar ratio of water to chlorine gas is between 0.01 and 2.0. Oxygen may also be present. The rejuvenation is typically carried out at 450.degree. C. (842.degree. F.).
U.S. Pat. No. 4,851,380, issued Jul. 25, 1989, to D. A. Van Leirsburg et al., describes a process for regenerating a sulfur contaminated reforming catalyst. The process includes a platinum agglomeration step which can be accomplished by contacting the catalyst with O.sub.2 between 800.degree. F. and 1200.degree. F. and an oxychlorination step. Pt redispersion by contacting an agglomerated catalyst with carbon monoxide and a sulfur free halogen acid gas.
U.S. Pat. No. 4,855,269, issued Aug. 8, 1989, to D. H. Mohr, discloses a platinum redispersion method comprising a wet oxychlorination step, followed by a nitrogen purge, followed by reduction.
U.S. Pat. No. 4,914,068 issued Apr. 3, 1990, to V. R. Cross, describes a process for the improved retention of the platinum dispersion following oxychlorination if the catalyst is cooled to 430.degree. C. or preferably less than 375.degree. C. prior to start of reduction. Coke was burned off the catalyst at 510.degree. C. prior to reduction.
Unfortunately, the presence of halogens such as chlorine or fluorine or their compounds during regeneration results in the formation of corrosive by-products such as HCl and NH.sub.4 Cl. These compounds can cause damage to equipment used in the reforming process such as heat exchangers, liquid/gas separators and reactor internals. The halogen containing compounds and their products necessary for platinum redispersion are highly toxic as well. Therefore, it is highly desirable to have available a catalyst regeneration procedure that does not include the use of a halogen containing gas.
U.S. Pat. No. 5,073,529, issued Dec. 17, 1991 to Miller et al., describes a process for regenerating a coke-deactivated reforming catalyst. A two-step carbon burn procedure is used wherein the catalyst is initially contacted with oxygen and an inert gas at a temperature ranging from 500.degree.-700.degree. F. During the second step, the catalyst is contacted with oxygen and an inert gas at a temperature ranging from 850.degree.-950.degree. F.